Interpretive Summary: This is an invited paper presented at the 2008 I.E. Melhus Graduate Student Symposium sponsored by the American Phytopathological Society. Citrus stubborn disease (CSD) is caused by infection with Spiroplasma citri, a phloem-limited, wall-less bacterium. The pathogen was examined in two commercial citrus orchards in Central California to optimize pathogen detection protocols; determine affect on crop production; and measure population genetic diversity. Best tissues for cultivation of the CSD pathogen were fruit columellae and receptacles. Detection of the CSD agent was better with DNA extraction and polymerase chain reaction assays from diseased tissue than cultivation. Yield from CSD trees was lower by 32% compared to the adjacent healthy trees and mis-shapen fruit further reduced production quality. Significant genetic diversity was ascertained but it was not correlated to time of collection (current vs. 20 years ago) or location. There was some indication, however, of host associations with non-citrus hosts. The impact of this work was its documentation of CSD incidence in some commercial groves as high as 85% and the significant damage it causes on yield and fruit quality. This data is being used to show that control measures to mitigate damage from CSD may be important in some groves.

Technical Abstract:
Although citrus stubborn disease (CSD), caused by the phloem resident mollicute Spiroplasma citri, is a significant threat to California citrus industry, our knowledge of its epidemiology is mostly anecdotal. We optimized multiple pathogen-detection protocols, measured disease incidence in two plots of commercial California groves, assessed pathogen impact on fruit quality and yield and evaluated genetic diversity among S. citri isolates. Fruit columellas and receptacles were more suitable than leaves or bark for bacterial cultivation. Using cultivation and S. citri–specific PCR for detection, the incidence of CSD in two sampled orchards, respectively, ranged from 46 to 85 % and 1 to 4 %, depending on the sampling technique. Yield and quality of fruits produced by trees that were mildly or severely CSD-symptomatic were compared to those of S. citri-free trees in one California orchard in 2006 and 2007. These infected trees had up to 32% lower yield and reduced fruit quality than S. citri-free trees. Using RAPD markers to compare 35 S. citri isolates isolated 20 years ago from the U.S. and Mediterranean region with 34 isolates recently collected from California, significant genetic diversity was identified but was not correlated with the time of collection or location. However, some host associations were observed. Our findings suggest that CSD incidence in the commercial groves evaluated could be as high as 84.8% and its impact on yield and fruit quality are significant.